In total shoulder arthroplasty, a glenoid implant is attached to a prepared glenoid or scapula, and a humeral implant is attached to a prepared humerus. The humeral implant usually includes a convex articular surface, at a proximal end thereof which engages and moves relative to a concave articular surface formed in the glenoid implant, although this arrangement is sometimes reversed so that the humeral implant includes the convex articular surface and the glenoid implant includes the convex articular surface. The ligaments and muscles of the shoulder surround the implants and maintain the humeral implant against the glenoid implant, while at the same time allowing relative movement therebetween.
Current anatomic prostheses for the proximal humerus generally fall into two types: stemmed prostheses and resurfacing prostheses.
Stemmed prostheses are quite common. Stemmed prostheses combine a hemispherical head replacement with a stem which extends into the shaft (diaphysis) of the humerus to anchor the prosthesis. Stemmed prostheses often require the removal of the entire hemisphere of humeral head bone, as well as drilling, reaming and/or broaching into the adjacent shaft of the humerus to seat the component. The hemispherical head component and stem are typically solid metal and can be of considerable weight. Stemmed prostheses also frequently require the surgeon to place the humeral head articular bearing surface in a position which is either fixed relative to the shaft of the humerus, or has modular adjustable connection mechanisms allowing partial adjustment between the placement of the hemispherical head component and the stem placed in the shaft of the humerus. This may not always match the actual anatomy of the patient, especially if deformity is present. Although many current prostheses provide for adjustments such as retroversion, offset, or neck-shaft angle, these adjustments are always limited to some degree, or constrained, by the stem to which the prosthetic humeral head is attached.
Resurfacing prostheses have a hollow hemisphere which rests on top of the humeral bone with a solitary peg or post in the humeral head for anchoring stability. Resurfacing prostheses have the advantage of resting directly on top of the bone of the upper humerus and do not have a stem that extends into the shaft of the humerus. Therefore the surgeon is free to place the prosthesis based on each individual patient's anatomy. Resurfacing prostheses also do not require the removal of the entire humeral head bone; simply the upper articular end is reshaped to accept the prosthesis sitting on top. The prosthesis itself acts as a surface cover, and the volume of bone underneath in the hemisphere remains. This preserves more of the patient's bone stock and if revision surgery is needed, allows for a much simpler re-operation because the shaft has not yet been violated.
The preservation of bone in the upper humerus with a resurfacing prosthesis may unfortunately become a disadvantage when the surgeon performs a total shoulder arthroplasty. In this operation, the surgeon also places a prosthesis into the glenoid cavity of the scapula. With the bone of the upper humerus still in the way, access to the glenoid may be very difficult and placing the glenoid prosthesis properly can be challenging.